The present invention relates to semiconductor optical amplifiers (SOAs) and in particular to linear optical amplifiers of the type formed by integrating an amplifier and a vertical cavity surface emitting laser (VCSEL).
Linear optical amplifiers of the type formed by integrating an amplifier and a VCSEL have been developed by Genoa Corporation and are described in their paper xe2x80x9cA single-chip linear optical amplifierxe2x80x9d by D. A. Francis, S. P. DiJaili and J. D. Walker (Optical Fibre Communications Conference, 17-22 March 2001, Anaheim, Calif., USA, paper PD13) which is incorporated herein by reference. The present invention builds on and extends the work described in that paper.
Optical communications networks use optical amplifiers as is well known in the art to increase the power of optical signals. As optical communications networks develop, increasing demands are required of such optical amplifiers in order that they can operate with increased data rates, in multi-wavelength environments, are smaller and can be easily integrated with other optical equipment and devices.
Existing optical amplifiers are problematic in many of these respects. For example erbium doped fibre amplifiers (EDFAs) are relatively expensive to manufacture, control and test because they contain many active and passive components. They also have a relatively large footprint. Semiconductor optical amplifiers avoid some of these problems, being smaller in size and less expensive to manufacture. However, they are non-liner and as a result cross-talk between channels occurs which is undesirable. Genoa Corporation address this problem of non-linearity by integrating a VCSEL into an SOA on an Indium Phosphide (InP) substrate. This is described in more detail below in the section headed SOA with Integrated VCSEL. However, the device proposed by Genoa Corporation is not suited for operation at speeds above 10 Gbps such as at speeds of 40 Gbps. In addition the saturation output power of the Genoa device is at around 8 dbm. However, for many applications, higher output powers of around 12 to 14 dbm and above are required.
An object of the present invention is to provide a semiconductor optical amplifier with integrated VCSEL that addresses or at least mitigates one or more of the problems noted above.
Further benefits and advantages of the invention will become apparent from a consideration of the following detailed description given with reference to the accompanying drawings, which specify and show preferred embodiments of the invention.
Semiconductor optical amplifiers (SOAs) are cheaper to manufacture, control and test than other types of optical amplifier such as erbium doped fibre amplifiers (EDFAs). However, SOAs are non-linear in the respect that the gain of an SOA is not constant for different input or output signal powers. This is a significant problem because cross-talk between channels occurs as a result. It is known that the gain of SOAs can be clamped by integrating a vertical cavity surface emitting lasr (VCSEL) with the SOA such that their active regions are shared. The present invention enables the physical length of such devices to be increased in such a manner that the saturation output power is increased whilst retaining the gain clamping effect. This is achieved by using two or more contact points on the device at which different drive currents are applied.
According to a first aspect of the present invention there is provided a semiconductor optical amplifier comprising:
a vertical cavity surface emitting laser (VCSEL) integrated with the semiconductor optical amplifier such that the active regions of those devices are shared;
two or more electrical contacts provided on the semiconductor optical amplifier and arranged such that an electrical current may be applied to those contacts in use such that power is applied to the VCSEL.
This provides the advantage that the range of input or output signal powers for which the gain of the SOA remains constant is increased. By using two or more contacts to add drive current to the VCSEL this is achieved. In addition the saturation output power of the SOA is increased. Preferably the amount of drive current applied at a particular one of the contacts is related to the position of the contact along the semiconductor optical amplifier. That is, larger drive currents are applied to contacts further along the length of the SOA in the direction of propagation of an optical signal through the SOA in use.
Preferably the semiconductor optical amplifier further comprises an electrically conducting layer which is separated into two or more regions by one or more electrically insulating areas and wherein each of said electrical contacts is provided on a different one of said regions. For example the electrically conducting layer can be a metallic layer applied to the surface of the SOA and into which a groove is etched between the two contacts in order to separate/isolate them from each other. This provides a simple and cost effective way in which the device can be manufactured.
Preferably the semiconductor optical amplifier has a length greater than about 1 mm. This provides the advantage that devices suitable for operation at high signal speeds are created.
Advantageously, the semiconductor optical amplifier is suitable for operation at signal speeds of greater than 10 Gbps and to have a signal output power of greater than 12 dbm.
According to another aspect of the present invention there is provided a method of amplifying an optical signal comprising the steps of:
passing the signal through a waveguide in a semiconductor optical amplifier with an integrated vertical cavity surface emitting laser (VCSEL); and
applying a current at two or more points along the semiconductor optical amplifier such that said current provides power to the VCSEL.
The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.